Background: Coordinated regulation of cell proliferation and cell death is essential to maintain homeostasis in multicellular organisms and deregulation of these processes can lead to tumor formation, as manifested in many human cancers. Recently, our laboratory and others have linked the function of tumor suppressor Merlin and Expanded in Drosophila to two signaling pathways that play critical roles in growth control: The Fat tumor suppressor pathway, and the Hippo pathway. These studies have revealed that growth regulation by Fat/Hippo/Merlin pathways is converged into a kinase called Warts, another tumor suppressor. Warts in turn acts by phosphorylating and thereby downregulating a transcriptional co-activator protein called Yorkie. Objective/Hypothesis: The principal hypothesis underlying this research is that Yorkie-dependent transcriptional regulation of target genes is crucial to the regulation of growth by multiple tumor suppressor pathways. Our principal objectives are to investigate molecular links from Fat/Hippo/Merlin pathways to growth control in Drosophila, which is mediated through Warts and Yorkie. Specific Aims: (1) Determine the molecular mechanism of regulation of Yorkie protein. (2) Identify DNA binding co-factors of Yorkie for transcriptional regulation of growth control. Study Design: The proposed experiments will take advantage of the many sophisticated genetic tools available in Drosophila for the in vivo analysis of gene function. For the first aim, we will employ molecular, genetic, and histological methods in Drosophila to examine the influence of Warts, Akt and 14-3-3 on the stability or subcellular localization of Yorkie. For the second aim, we will identify DNA binding co-factors that interact with Yorkie to regulate the transcription of downstream genes by genetic modifier screening and yeast two-hybrid approaches. Relevance: Given that many upstream components of Warts and Yorkie in Fat/Hippo/Merlin pathways are conserved between human and fruit fly, and that the pathological symptoms of most of human cancers are uncontrolled overgrowth of tissues, this study will help us to understand underlying molecular mechanism in growth control by several tumor suppressors in Fat/Hippo/Merlin pathways in Drosophila. Drosophila has proven to be an outstanding model for characterizing many fundamental aspects of animal physiology and development, including the regulation of tissue growth, and we expect that insights into the molecular basis of growth regulation by several tumor suppressor pathways in Drosophila will be directly relevant to understanding the molecular basis for growth regulation by human tumor suppressors.